The Behaviour of radiocaesium trapped and deposited in the reservoir after the Fukushima nuclear accident

Funaki, Hironori ; Hagiwara, Hiroki  ; Tsuruta, Tadahiko  

Radiocaesium from the Fukushima Nuclear accident fallout is currently the main air dose rate controlling contaminant and has a tendency to be strongly absorbed on soil particles. Understanding the behaviour of a contaminated soil in the environment of Fukushima, especially at forest, aquatic and agricultural areas, is crucial for evaluating the prolonged exposure. The Japan Atomic Energy Agency has been examining the mass balance of in- and outflow of the soil particles at the Ogi reservoir with a 4.2 km watershed in Kawauchi-mura, Fukushima, since fall 2012. The high resolution bathymetric survey using multibeam sonar and the sampling of bottom sediments using gravity samplers were performed. The results show the homogenization of the vertical Cs distribution (29-48 kBq kg, dry weight) in the fine-grained bottom sediments until the depth of 22 cm near the intake of the reservoir. Below the 22 cm depth, the Cs activities exponentially decreased with depth. In the upper part of the reservoir, the fine-grained sediments were ca. 7 cm thickness and overlie coarse-grained sediments, which prevent the penetration of gravity sampler. These results indicate that the contaminated fine-grained sediments tend to be deposited thickly at the intake, where corresponds to the deepest part of the reservoir. The total mass of sediment from the catchment area has been estimated to approximately 1,200 tons in the past two years.